In order to meet demands for weight reduction and shortening of construction period, it is strongly desired to increase the strength of a variety of wire ropes such as power transmission cables and suspension bridge cables. As the strength of a wire rope increases, a demand for increasing the strength of a steel wire used as a material of the wire rope is increasing.
Steel wires are generally manufactured by subjecting a steel wire rod to a patenting process and then drawing the steel wire rod. A plurality of the thus obtained steel wires are twisted together by stranding to form a wire rope.
The largest problem in increasing the strength of a steel wire is to secure ductility and suppress a crack (delamination) occurring in the longitudinal direction of the steel wire at the time of twisting such as stranding.
Examples of conventional techniques for suppressing delamination include the techniques described in Patent Document 1 and Patent Document 2.
Patent Document 1 describes a PC steel wire which achieves both high strength and longitudinal crack (delamination) prevention by appropriately controlling the residual stress and yield ratio of the surface.
Patent Document 2 describes a technique of preventing sticking of N atoms to the dislocation in the structure of a steel wire as much as possible, improving the ductility of the steel wire, and preventing occurrence of delamination.
In addition, Patent Document 3 describes high-strength wire rod excellent in delayed fracture resistance which is composed of a steel containing C: 0.5 to 1.0% (meaning % by mass, the same applies hereinafter), in which the area ratio of the pearlite structure is 80% or more by suppressing the generation of one or more structures of pro-eutectoid ferrite, pro-eutectoid cementite, bainite, and martensite, and which has a strength of 1,200 N/mm2 or more and excellent delayed fracture resistance by strong wire drawing.
Patent Document 4 describes a wire rod in which an area of 97% or more of the cross section perpendicular to the longitudinal direction of the wire rod is occupied by the pearlite structure, and an area of 0.5% or less of a central region of the cross section and an area of 0.5% or less of the first surface layer region of the cross section are occupied by a pro-eutectoid cementite structure.
Patent Document 5 describes a wire rod in which the main phase of the structure is pearlite, the AlN content is 0.005% or more, and in a maximum extreme value distribution of the diameter dGM of AlN represented by the geometric mean (ab) ½ of a length a and a thickness b, the percentage of AlN with a dGM of from 10 to 20 μm is 50% or more based on the number.
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2005-232549
Patent Document 2: JP-A No. 2005-126765
Patent Document 3: JP-A No. H11-315347
Patent Document 4: WO2011/089782
Patent Document 5: Japanese Patent 5833485